077 page change stamps: Flow PT REST finding

Q What tells a client that a page's layout changed: an updated_at on Page or PageSetting, or an EventLogEntry row for the save?

Endpoint GET /schema/Page/fields ; GET /schema/PageSetting/fields ; POST /entity/event_log_entries/_search

Docs claim Silent.

Actual

/schema/Page/fields: 24, among them updated_at date_time, updated_by entity, created_at, created_by,
  last_accessed, hits_last_month, accesses_by_current_user, current_user_can_see, shared, system_owned
/schema/PageSetting/fields: 6: cached_display_name, created_at, id, page, settings_json, user
PageSetting ?fields=updated_at,updated_by,created_at,created_by -> attributes [created_at], nothing else
PageSetting [["updated_at","is_not",null]] -> 400 "API read() PageSetting.updated_at doesn't exist."

event types starting Shotgun_Page, site-wide:
  Shotgun_Page_Change 8665   Shotgun_Page_New 4497   Shotgun_Page_Retirement 3536
  Shotgun_PageSetting_Change 8534   Shotgun_PageSetting_New 4739
Shotgun_PageSetting_Change, newest 500: attribute_name null on all; entity {Page: 369, null: 131}
  meta keys {(changes, friendly_changes, type): 233, (page_setting_history_id): 221,
             (changes, friendly_changes, page_setting_history_id, type): 41, (): 5}
  a change entry: {"path": ["body", "list_content"], "type": "SG.Widget.NewGrid", "setting": "columns"}
    also seen: a view_name key (38), an added key in place of setting (4)
  friendly_changes: "On Grid changed panel filters\nOn Grid added column \"Project\" (project)"
  /schema types naming History: none, so page_setting_history_id resolves to nothing over REST
Shotgun_Page_Change: attribute_name set (name on the newest 50), meta old_value/new_value as usual

Page.updated_at against the page's newest event:
  newest Shotgun_PageSetting_Change at 18:45:27Z   Page.updated_at 18:45:27Z
  newest Shotgun_Page_Change at 18:43:26Z          Page.updated_at 18:43:26Z   (3 pages)

Teaches

signal	on	tells you
`Page.updated_at`, `updated_by`	Page	the page or its layout changed; it moved to the second of a layout save
`PageSetting.updated_at`	none	the field does not exist: `?fields` drops it, a filter 400s
`Shotgun_PageSetting_Change`	EventLogEntry	which widget and which setting changed, never the new value
`Shotgun_Page_Change`	EventLogEntry	an attribute change on Page, with `old_value`/`new_value`

Cache a layout keyed on Page.id plus Page.updated_at; one ?fields=updated_at read tells you whether to refetch the tree.
PageSetting.created_at does not move on an edit, so it cannot stand in for a change stamp.
A settings event names the Page under entity on 369 of 500; on the other 131 entity is null and nothing in meta names the row or the page, so the event log cannot tie every save to a page.
meta.changes lists {path, type, setting} per change. Diff the trees yourself for values.
A script's own PUT on settings_json logged a different shape (probe 078): an attribute_change whose old_value and new_value are the whole JSON as strings.